1. Field of the Invention
The present invention relates to a printing apparatus performing color printing by thermal transfer.
2. Description of the Related Art
Printing apparatuses based on a thermal transfer technology such as a dye-sublimation method, a thermal fusion method, or a heat-sensitive method have been widely used. A thermal transfer printing apparatus based on, for example, the dye-sublimation method is configured to perform color printing by heating ink to sublimation, so it can adjust the amount of ink by precisely controlling heat quantity, resulting in an image quality nearly as high as that of a photograph.
As an example of such a printing apparatus based on the thermal transfer technology, a line thermal printer has been proposed. The line thermal printer includes a thermal head in which a plurality of heater elements (resistive elements) are linearly arranged. Electric power is selectively applied to the plurality of heater elements in the thermal head depending on a gradation level, and the resulting thermal energy is used to print an image on a recording medium such as printing paper or a printing film. More specifically, the thermal head is configured to descend toward a platen arranged to oppose the thermal head, thereby nipping an ink ribbon and the recording medium conveyed between them. Electric power is selectively applied to the heater elements in the thermal head and the ink on the ink ribbon is heated to sublimation and transferred onto the recording medium, thereby performing the printing on the recording medium.
At this time, the thermal transfer of the ink onto the recording medium is performed on a unicolor basis. Therefore, the line thermal printer controls conveyance of the recording medium to reversely transfer the recording medium to a position at which the transfer started every time color of ink to be thermally transferred is changed.
A forward conveyance and a reverse conveyance of the recording medium are performed by various conveyance rollers arranged in a conveyance path rotating forward or backward in a state of nipping the recording medium. As a drive source of the rollers, a stepper motor is generally used. This is because the use of the stepper motor facilitates drive control and enables a precise drive operation.
That is, the line thermal printer generally prints an image by driving the stepper motor based on control data in which the number of pulses, a drive frequency, and the like are set in advance, conveying the medium precisely (forward and backward) by the drive of the stepper motor, and driving the thermal head at the same time. As a result of employing the stepper motor driven by the control data, the line thermal printer may not detect a transfer start position and a transfer end position of the recording medium to be performed with respect to each color anymore to realize a sequential conveyance operation, thereby reducing a printing time.
The thermal printer that performs the color printing by thermal transfer using a plurality of colors may be disadvantageous in that the recording medium is extended or shrunk in a sub-scanning direction by the heat generated at the time of directly heating the recording medium for the thermal transfer of each color. Generation of such an extension and a shrink may cause a degradation of quality of a formed image because of registration deviations among images formed on the recording medium in respective colors. In this regard, there is proposed a technology of reducing the registration deviations caused by the extension and the shrink of the recording medium by changing the transfer start position in a sub-scanning direction depending on the amount by which the recording medium extends or shrinks at the time of the thermal transfer of each color in the thermal printer when the thermal transfer is performed in the plurality of colors (see, for example, Japanese Patent No. 3075885).